An eco-sustainable green approach for the synthesis of propargylamines using LiOTf as a reusable catalyst under solvent-free condition

Article abstract of DOI:10.1007/s12039-012-0352-z

An efficient process has been developed for the synthesis of propargylamines via a three-component coupling reaction of aldehyde, secondary alicyclic amine and alkyne (A³) under solvent-free condition using lithium triflate (LiOTf) as expeditious reusable catalyst. This one-pot transformation generates one C-C and one C-N bond, which presumably proceeds by lithium acetylide as well as formation of iminium ion in situ and then undergoes nucleophilic addition to the iminium ion to give the propargyl amine. The solvent-free condition, easy recovery of the catalyst, simple, user-friendly and quantitative yield in short time renders the protocol economic and reasonable.

Full text of DOI:10.1007/s12039-012-0352-z

c
J. Chem. Sci. Vol. 125, No. 1, January 2013, pp. 101–107. ꢀ Indian Academy of Sciences.
An eco-sustainable green approach for the synthesis of propargylamines
using LiOTf as a reusable catalyst under solvent-free condition
SOMESHWAR D DINDULKAR, BAEK KWAN, KWON TAEK LIM and YEON TAE JEONG∗
Department of Image Science and Engineering, Pukyong National University, Busan 608-739,
Republic of Korea
e-mail: ytjeong@pknu.ac.kr
MS received 2 March 2012; revised 5 April 2012; accepted 26 April 2012
Abstract. An efficient process has been developed for the synthesis of propargylamines via a three-
component coupling reaction of aldehyde, secondary alicyclic amine and alkyne (A3) under solvent-free con-
dition using lithium triflate (LiOTf) as expeditious reusable catalyst. This one-pot transformation generates one
C–C and one C–N bond, which presumably proceeds by lithium acetylide as well as formation of iminium ion
in situ and then undergoes nucleophilic addition to the iminium ion to give the propargyl amine. The solvent-
free condition, easy recovery of the catalyst, simple, user-friendly and quantitative yield in short time renders
the protocol economic and reasonable.
Keywords. Lithium triflate (LiOTf); three-component coupling (A3); propargylamine; C–H activation;
solvent-free condition.
1. Introduction
as aminolysis of oxiranes, glycosylation, dithioacetal-
ization of carbonyl compounds, tetrahydropyranylation
of alcohols/phenols and acetylation of alcohols as well
as diacetylation of aldehydes have been reported.4a−b
Elegant features like low toxicity, air/water compa-
tibility, operational simplicity, and remarkable ability
to suppress side reactions in acid sensitive sub-
strates makes it valuable and advantageous catalyst in
synthetic processes. Many LiOTf catalysed reactions
such as carbon–carbon, carbon–heteroatom bond and
heteroatom–heteroatom bond forming reactions have
been described recently.4c−e
Three-component coupling of an aldehyde, alkyne,
and amine (A3-coupling) via activation of a terminal
alkyne C–H bond by transition metal catalysts is a
reaction of great interest in organic chemistry.5a−b The
formed propargylamines are synthetically and biologi-
cally important because of a wide range of applica-
tion such as optical devices, valuable building blocks
for the synthesis of many different natural product syn-
thesis, various nitrogen compounds such as pyrroles,6a
β-lactams,6b pyrrolidines,6c and prized precursors for
therapeutic drug molecules.6d−k Furthermore, they pos-
sess highly potent, irreversible, and selective inhibitory
activity toward monoamine oxidase B (MAO B),6g
while N-benzylpropargylamines show aldehyde dehy-
drogenase inhibitory activity.6h Very recently, a series
of propargylamines such as rasagiline [N-propargyl-
(1R)-aminoindan] have been employed as a selective
Multicomponent reactions, especially those run under
solvent-free conditions, have been attracted increas-
ing research interest from chemists in recent years.1a−c
Most of the organic reactions are conventionally car-
ried out in solvent media. However, to minimize
the environmental pollution caused by solvents, the
chemists have been showing more concern for devel-
oping environment-friendly synthetic procedures. This
initiative, aided with the recent development of new
strategies in solid–solid reactions has prompted them to
develop sufficiently valuable methodologies to achieve
organic synthesis under solvent-free condition.1d−f
Especially, multicomponent reactions that provide
poly-functionalized heterocyclic scaffolds in single
operation and in stereo-specific manner are of great
importance in synthetic organic and medicinal chem-
istry.2a−b Metal-catalysed multi-component reactions
(MCRs) are a classical synthetic tool to access com-
plex molecules from simple precursors via a one-
pot procedure, as they exhibit economical, practical
and efficient ways for chemical synthesis and have
become increasingly important in the organic synthetic
research.3a−c Lithium triflate (LiOTf) has emerged as a
highly efficient, mild, neutral Lewis acid and reusable
catalyst for a variety of organic transformations such
∗For correspondence
101

102
Someshwar D Dindulkar et al.
To the best of our knowledge, there is no report avai-
irreversible monoamine oxidase (MAO)-B monother-
apy in early Parkinson’s disease or as an adjunct ther- lable in the literature on the use of LiOTf as reusable
apy in more advanced cases.7a−b Also they inhibit catalyst in the synthesis of propargylamine using sec-
acetylcholinesterase (ChE) enzyme and are brain selec- ondary amine, aldehyde and acetylene under solvent-
tive type A and B MAO inhibitors and have been free conditions. The main focus of our study was to
shown to improve memory impairments in scopolamine develop a solvent-free reaction for the enhancement
treated rats. Its sisomer, TV-3326 and TV-3279 which of reaction rate with a cleaner conversion, leading to
is also a ChE (figure 1) has no MAO inhibitory activity an economical protocol for this one-pot three compo-
but depicted similar action in scopolamine impairment nent (A3) system. In this paper, we wish to disclose
test.7c
the development of a new, mild and truly efficient one-
Due to importance of propargylamines in organic pot three component (A3) synthesis of propargylamines
synthesis and lack of efficient and convenient meth- using alicyclic amines, aldehyde and alkynes in the
ods for their synthesis, there is an imperative need presence of LiOTf as reusable catalyst in air under
to develop competent synthetic procedure that would solvent-free condition (scheme 1).
be a useful protocol for the synthesis of this impor-
tant precursor. As a consequence, many efforts have
been devoted towards these valuable structures. In
this context, many Lewis acid catalysts of tran-
2. Experimental
2.1 Typical procedure for the A3 coupling reaction
sition metals reported earlier, such as Zn(OAc)2,
FeCl3, CuI, Si(CH2)3SO3CuCl, ZnOTiO2 (nano pow-
der), AgI, AuBr3, Au-HS/SO3H-PMO (Et) in vari-
ous toxic organic solvents such as acetonitrile, DMF,
toluene, and DCM under an inert atmosphere have been
employed.8a−i The use of recyclable catalysts under
solvent-free condition in organic transformations have
become an interesting area of research in the field
of green chemistry.9a−b Keeping in view the limita-
tions associated with reported methods, there is a high
demand for the improved synthesis of propargylamines
via the activation of a terminal alkyne C–H bond using
transition metal-catalyst. As an extension of our contri-
butions in the field of environmentally friendly Lewis
acid mediated one-pot synthesis,10a−d we studied for the
development of highly efficient protocol for this three-
component coupling of aldehydes, secondary amines,
and alkynes (A3-coupling) in terms of cost effective-
ness, reduce in reaction time with better yields.
A mixture of benzaldehyde 106 mg (1 mmol), piperi-
dine 85 mg (1 mmol), phenylacetylene 132 mg
(1.3 mmol) and LiOTf 31 mg (20 mol%) was taken in
a round-bottom flask and stirred at 90◦C temperature.
After completion of the reaction (monitored by TLC),
the mixture was allowed to cool at room temperature
and the reaction was subsequently quenched with water
(25 mL) followed by the extraction with ethyl acetate
(20 mL). The organic layer was separated, washed
with saturated NaHCO3 (20 mL) and water (15 mL)
and dried over anhydrous Na2SO4. Evaporation of the
solvent under reduced pressure gave the crude product,
which was chromatographed on silica gel to afford
1
the pure product. The H and 13C NMR data of the
known compounds are in good agreement with those
in the literature. All new compounds were completely
characterized by their analytical and spectral data.
Spectroscopic data for new compounds are given and
scan files provided as supporting information.
O
2.2 4-(1-(4-Chlorophenyl)-3-phenylprop-2-ynyl)
thiomorpholine (table 3, entry 11)
N
O
HN
HN
Pale yellow solid, Mp. 56–58◦C, Yield 92%, FT-IR
(cm−1): 684, 748, 818, 950, 1011, 1088, 1214, 1276,
1310, 1401, 1445, 1486, 1593, 1658, 2824, 2908, 3057.
1H NMR (400 MHz, CDCl3): δ = 7.55(d, J = 8.0 Hz,
2H), 7.51–7.49 (m, 2H), 7.31–7.29 (m, 5H), 4.73 (s,
1H), 2.86–2.79 (m, 4H), 2.67–2.62 (m, 4H); 13C NMR
(100.5 MHz, CDCl3): δ = 143.1, 131.7, 131.0, 129.6,
128.4, 128.3, 128.2, 122.6, 88.8, 84.2, 62.0, 51.7, 28.2.
HRMS: m/z calculated: 327.0871, found: 327.0851.
Rasagiline
TV-3326
O
N
O
HN
TV-3279
Figure 1. Propargylamine inhibitors of monoamine oxi-
dase (MAO)-B.

Synthesis of propargylamines using LiOTf under neat condition
103
X
N
LiOTf (20 mol%)
+
O
X
N
H
H
neat, 90 o
C
R
-H2O
R
H
X = CH , O, S
2
Scheme 1. Lithium trifluoromethanesulphonate catalysed three component synthesis of propargylamines.
2.3 1-(1-(2, 3-Dihydrobenzo[b][1,4]dioxin-7-yl)-3- (m, 4H), 2.73–2.62 (m, 4H); 13C NMR (100.5 MHz,
phenylprop-2-ynyl)piperidine (table 3, entry 14)
CDCl3): δ = 143.2, 131.8, 131.2, 128.3, 123.0, 121.4,
117.3, 116.9, 88.3, 85.1, 64.4, 62.2, 51.7, 28.3. HRMS:
m/z calculated: 351.1350, found: 351.1296.
Brown solid, Mp. 58–60◦C, Yield 89%, FT-IR (cm−1):
690, 755, 780, 860, 909, 1000, 1088, 1066, 1113, 1253,
1285, 1453, 1503, 1590, 2820, 2853, 2926, 3055. H
1
NMR (400 MHz, CDCl3): δ = 7.50–7.47 (m, 2H),
7.30–7.25 (m, 3H), 7.16 (s, 1H), 7.08 (d, J = 8.4 Hz,
1H), 6.83 (d, J = 8.8 Hz, 1H), 4.67 (s, 1H), 4.20 (s,
4H), 2.54 (unresolved dt, 4H), 1.59–1.55 (m, 4H), 1.42–
1.39 (m, 2H); 13C NMR (100.5 MHz, CDCl3): δ =
143.1, 131.7, 131.0, 128.2, 122.9, 121.4, 117.3, 116.8,
88.1, 85.2, 67.0, 64.2, 61.3, 49.7, 14.1. HRMS: m/z
calculated: 323.4235, found: 327.1730.
3. Results and discussion
Initially, to find out the suitable catalyst for this A3 cou-
pling, the optimization of various reaction parameters
like different metal Lewis acid catalysts, temperature,
and solvent was carried out (tables 1 and 2, respec-
tively). In search of highly effective catalyst, we ini-
tially performed a reaction in the absence of catalyst in
toluene at 90◦C (table 1, entry 1). It was observed that
no product was formed in the absence of catalyst even
after 12 h. From this study, we concluded that a catalytic
system is mandatory for this A3 coupling. To ensure
the reusability of catalyst, a Cu–Sn nano alloy (200
mesh, 100 mg) was employed as a catalyst which unfor-
tunately resulted in poor yield even after a long reaction
time. It is important to mention that when we increased
amount of Cu–Sn catalyst 200, 300 and 400 mg respec-
tively, the results indicated that 400 mg Cu–Sn alloy
was providing higher isolated yields in shorter times as
compare to 100 mg of Cu–Sn (table 1, entry 5). We fur-
ther experimented with different metal Lewis acid cata-
lysts such as zinc triflate, lithium triflate, magnesium
triflate and BF3·SiO2, Cu(OTf)2.SiO2, Zn(OTf)2.SiO2
catalyst (table 1), and found that among all screened
catalysts, lithium triflate gave the best yield in least
reaction time (table 1, entry 10).
2.4 4-(1-(2,3-Dihydrobenzo[b][1,4]dioxin-7-yl)-3-
phenylprop-2-ynyl)morpholine (table 3, entry 15)
Pale yellow solid, Mp. 63–65◦C, Yield 94%, FT-IR
(cm−1): 697, 762, 823, 885, 903, 988, 1066, 1092, 1112,
1199, 1251, 1381, 1424, 1456, 1501, 1588, 1663, 2360,
1
2749, 2808, 2851, 2928, 3052. H NMR (400 MHz,
CDCl3): δ = 7.49–7.46 (m, 2H), 7.29–7.28 (m, 3H),
7.15 (d, J = 1.8 Hz, 1H), 7.08 (dd, J1 = 1.8 Hz, J2 =
8.4 Hz, 1H), 6.82 (d, J = 8.4 Hz, 1H), 4.66 (s, 1H),
4.19 (s, 4H), 3.74–3.69 (m, 4H), 2.64–2.54 (m, 4H); 13C
NMR (100.5 MHz, CDCl3): δ = 143.1, 131.7, 131.0,
128.3, 122.9, 121.4, 117.3, 116.8, 88.1, 85.1, 67.0, 64.2,
61.3, 49.7, 14.1. HRMS: m/z calculated: 335.3963,
found: 335.1519.
Optimization of LiOTf as best catalyst for this
reaction (table 1, entry 10), was followed by solvent
optimization for enhancing yields, we also tested dif-
ferent solvents such as EtOH, CH3CN, water, PEG and
[BMIM]Cl as polar solvent at 90◦C. Among the vari-
ous solvents tested, toluene was found to give the best
result in which the reaction proceeded smoothly giving
the maximum yield in minimum time. It is noteworthy
here that when water was used as a solvent, very low
yield was obtained even after a prolonged reaction time
2.5 4-(1-(2,3-Dihydrobenzo[b][1,4]dioxin-7-yl)-3-
phenylprop-2-ynyl)thiomorpholine (table 3, entry 16)
Pale yellow solid, Mp. 120–122◦C, Yield 92%, FT-IR
(cm−1): 700, 763, 827, 888, 953, 1065, 1140, 1200,
1253, 1283, 1424, 1452, 1501, 1584, 2830, 2873, 2918,
2952. 1H NMR (400 MHz, CDCl3): δ = 7.51–7.49 (m,
2H), 7.31 (t, J = 3.3 Hz, 3H), 7.15 (d, J = 1.8 Hz,
1H), 7.08 (dd, J1 = 1.8 Hz, J2 = 8.4 Hz, 1H), 6.82 (d,
J = 8.4 Hz, 1H), 4.70 (s, 1H), 4.23 (s, 4H), 2.90–2.81

104
Someshwar D Dindulkar et al.
Table 1. Optimization of the catalyst.a
O
H
N
Catalyst
Toluene, 90 oC
+
+
N
H
Entry
Catalyst (10 mol%)
Time (h)
Yieldb
1
2
3
4
5
6
7
8
–
12
12
8
8
8
12
12
08
12
06
12
5
No product
Cu–Sn (200 mesh, 100 mg)
Cu–Sn (200 mesh, 200 mg)
Cu–Sn (200 mesh, 300 mg)
Cu–Sn (200 mesh, 400 mg)
Cu(OTf)2.SiO2
54
63
70
73
67
52
37
70
78
63
84
84
Zn(OTf)2.SiO2
BF3·SiO2
9
Zinc triflate
10
11
12
13
LiOTf
Magnesium triflate
LiOTf (20 mol%)
LiOTf (30 mol%)
5
aAll reactions were run with benzaldehyde (1 mmol), piperidine (1 mmol), phenylacetylene (1.3 mmol) and catalyst (10 mol%)
in 1 ml of toluene at 90◦C in air; bIsolated yields
(table 2, entry 4). Moreover, when we used [BMIM]Cl (89%), probably due to high concentration of reac-
as polar medium, no product formation was noticed tants under neat condition (table 2, entry 7). Conse-
even after 12 h of reaction time (table 2, entry 6). quently, the optimum 20 mol% loading of the LiOTf
It is important to report that when the reaction was provided the maximum yield in minimum time (table 2,
performed under solvent-free conditions, reaction was entry 7), but loading of higher amount of catalyst nei-
completed in shorter time with increased in yield ther increased the yield nor reduced the reaction time
Table 2. Optimization of the reaction conditions with different solvents.a
Entry
Solvent
LiOTf (20 mol%)
Temp. (◦C)/time (h)
Yieldb
1
2
3
4
5
6
7
8
Toluene
Acetonitrile
Ethanol
Water
PEG
[BMIM]Cl
Neat
Neat
Neat
Neat
Neat
–
–
–
–
–
–
–
90/5
90/12
90/12
100/24
100/24
100/12
90/45 min
90/1
84
80
25
<10
56
No product
89
90
90
72
86
78
72
30 mol%
40 mol%
05 mol%
IInd cycle
IIIrd cycle
IVth cycle
9
90/1
90/2
90/1
90/1
10
11
12
13
Neat
Neat
90/1
aPhenylacetylene:piperidine:benzaldehyde (1.3:1:1).
bIsolated yields

Synthesis of propargylamines using LiOTf under neat condition
105
(95% catalyst was recovered). The recovered LiOTf
was dried at 90–100◦C for 12 h and tested up to three
more reaction cycles. Recycling and reuse of the cata-
lyst showed minimal decreases in yields (table 2, entries
11, 12 and 13), thus proving the catalyst’s reusabi-
lity. Figure 2 shows the variation of yield of 1-(1,3-
diphenylprop-2-ynyl)piperidine (table 2, entries 10, 11,
12 and 13) with the number of times the LiOTf were
recycled and reused. We have determined that 20 mol%
LiOTf is the optimum catalytic amount under solvent-
free condition for this system. Once the effective
catalytic amount of catalyst was proven, general appli-
cability and efficiency of developed protocol were
investigated using aldehydes with different functional
groups, various secondary amine and alkynes for the
synthesis of different propargylamines under solvent-
free condition (table 3).
Figure 2. The reuse of LiOTf in the synthesis of compound 5.
Among all screened secondary amines, in which ali-
cyclic amines like morpholine and thiomorpholine gave
better yield in short time (table 3). The only exception
was the reaction with 4-nitrobenzaldehyde, in this case
very low yield was obtained after long reaction time
due to strong electron withdrawing substituents (table 3,
entry 12). In addition, 3-hydroxy benzaldehyde also
showed comparably good yields (table 3, entry 13). It
is important to note that aliphatic aldehyde also showed
high reactivity with good yield in short reaction time
(table 2, entry 8 and 9), however the yield decreased by
using the lower amount of LiOTf (table 2, 10).
We were also interested to study the reusability of the
LiOTf catalyst for at least three more cycles. Accord-
ingly, after the first run with 89% yield, the catalyst
was recovered from aqueous layer of first washing of
the work-up. After evaporation of water under reduced
pressure, the resulting solid residue was precipitated
out in acetonitire to give the corresponding pure LiOTf
Table 3. Synthesis of different propargylamines under solvent-free condition.a
Entry
R
X
Time (min)
65
Yieldb
84
O
1
N-CH3
H
O
2
3
N-CH3
N-CH3
70
45
82
83
H
MeO
Br
O
H
O
4
5
6
N-CH3
CH2
30
60
40
86
89
91
H
Cl
O
H
O
H
CH2
Cl

106
Someshwar D Dindulkar et al.
Table 3. (continued).
Entry
R
X
Time (min)
50
Yieldb
87
O
O
7
8
CH2
H
Br
Cl
Br
H
O
25
93
O
H
9
O
S
S
25
40
30
88
86
92
O
10
11
H
O
H
H
Cl
O
12
13
14
15
CH2
CH2
CH2
O
180
70
58
79
90
94
O2N
HO
O
H
O
O
O
O
O
40
H
H
30
O
O
O
16
17
S
S
50
45
92
91
H
O
O
H
aReaction condition: aldehyde (1 mmol), alicyclic amine (1 mmol), phenylacetylene (1.3 mmol) and catalyst (20 mol%) at
90◦C under neat condition in air
bIsolated yields
(table 3, entry 17). From the study of different screened
On the basis of previously well-documented
aldehydes and secondary amines, it is concluded that results,8a−c it is postulated that the A3 coupling reaction
not only electronic effect of aldehyde substituent affect proceeds by terminal alkyne C–H bond activation. The
the rate and yield of reaction but also the nature of LiOTf activate the C–H bond of alkyne give the lithium
secondary amine matters for this A3 coupling.
acetylide as well as formation of iminium ion from

Synthesis of propargylamines using LiOTf under neat condition
107
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X
N
Ph
HC
LiOTf
H2O +
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Ph
-OHTf
Li
Ph
-OTf
X
N
X
O
+
-HO-
R
H
N
H
R
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an aldehyde and secondary amine in situ. The lithium
acetylide intermediate reacts with the iminium ion to
give the corresponding propargylamine and releases
the LiOTf catalyst for further reaction. A postulated
mechanism is shown in scheme 2.
4. Conclusion
In conclusion, we have developed an efficient, sim-
ple and economic protocol for the three-component
(A3) reaction of terminal alkynes, alicyclic amines, and
aldehydes under solvent-free condition using LiOTf as
expeditious and reusable catalyst. The main advantages
of this method lie in the usage of LiOTf catalyst under
solvent-free condition, which include shorter reaction
time, simple work-up and enhanced yield.
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Supporting Information
For supporting information (figures S1–S8), see
www.ias.ac.in/chemsci website.
Acknowledgement
This research work was supported by Second Phase of
Brain Korea (BK21) Program.
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